D−π−A Strategy to boost dielectric breakdown strength of polyimide insulation

Polyimide (PI) possesses multiple advantages including excellent thermal stability, mechanical property and chemical resistance, which has been considered as one of the promising candidates for high-performance electrical insulation materials. Optimizing the polymer structure such as extending the band gap can yield high intrinsic breakdown strength, attributed to the inhibition of impact ionization. However, in most cases, large band gap is achieved at the cost of reducing the backbone coplanarity and rigidity or the intermolecular interaction, resulting in undesirable thermal and mechanical performance. Herein, we introduce an effective strategy to produce thermally stable and mechanically strong polyimide insulation with high breakdown strength through introducing aromatic donor groups into the main chains to construct a D−π−A structure. On the one hand, the formation of D−A effect increases the materials polarization and enhances dipole-induced scattering. On the other hand, enhanced intermolecular interaction via π−π stacking decreases free volume in PIs. Then, increased conjugation degree of the backbone and high packing density between the polymer chains benefit to thermal stability and mechanical properties. Based on this strategy, N-arkyl carbazole groups are preferred, in which the soft alkyl side groups can occupy the pores between the polymer chains to further decrease free volume. As a result, we afford polyimide insulation with significantly improved breakdown strength from 291 kV mm−1 to 570−632 kV mm−1, as compared to the reference PI ODA-PMDA. Simultaneously, the resultant PIs, such as PI-Bu, preserve high Td5% above 524 ℃, high Tg above 364 ℃ and tensile strength above 80 MPa.